Oxidation states and redox reactions
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At what concentration does sulphuric acid act as an oxidising agent? (or where should iIlook to find this out - I searched through Google!)
Does hydrochloric acid act as an oxidising agent at any concentrations?
Jenkin writes
There is no ‘hard and fast’ answer here. The usual, and probably the most useful, way of looking at this is to think of concentrated sulphuric acid as an oxidising agent but dilute as non-oxidising.
The concentrated acid, which consists essentially of H2SO4 molecules and is not ionised, will oxidise metals such as copper and zinc to their ions, whilst itself being reduced to sulphur dioxide, sulphur or even hydrogen sulphide. The products depend on the reaction temperature, and also on whether any water is present. We are here thinking of a few % of water, not of significant dilution. There are, of course, many other oxidations that can be brought about by concentrated sulphuric acid, such as bromides and iodides to bromine and iodine respectively, carbon to carbon dioxide and sulphur to sulphur dioxide.
HCl is not an oxidising agent: it can itself be oxidised to chlorine by powerful oxidising agents such as potassium manganate(VII).
Some might argue that when a metal such as magnesium reacts with dilute sulphuric or hydrochloric acid, forming a solution of magnesium ions, that this is oxidation because Mg atoms have lost electrons. But here the real reaction is between magnesium and hydrogen ions, not with the un-ionised acid.
So perhaps the answer is that when the acid is not ionised it is an oxidising agent. But there is no sharp cut-off between un-ionised and completely ionised H2SO4; there will be concentrations at which the acid is a mixture of the two. HCl is different because it is made by dissolving hydrogen chloride gas in water; as the gas dissolves it reacts with the water to form hydrogen and chloride ions, so it is in the ionised form at all concentrations.
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updated: 11 November 2007
